A continuing concern in the aerospace industry is the design of aircraft which safely dissipate natural electric discharges. In a previous era when aircraft were primarily constructed from materials having uniform electrical conductivity characteristics, such as metal, the problem of safely dissipating electrical discharges was not particularly troublesome. However, with the increasing use of composite materials such as graphite fiber/epoxy resin in conjunction with metal structural elements, safe dissipation of electrical discharges in aircraft has become increasingly difficult to achieve.
Modern composite aircraft typically utilize woven sheets of graphite fiber material or tape which are impregnated with a resinous material such as epoxy. These sheets are then bonded together so as to form a lightweight laminate having considerable structural strength.
It is well known that the uppermost layer of the laminate may be provided with a quantity of metal to prevent electrical discharges from traveling through deeper layers of the laminate and to dissipate the discharge throughout the surface of the aircraft skin. As is also known from my U.S. patent application Ser. No. 000,926, titled "Metallic Conduction Method and System for Joined Sections of Composite Structures," different quantities of metal per unit area may be advantageously deposited in the uppermost layer of different areas of the aircraft to both maximize lightning protection and minimize the weight of the aircraft. For example, 200 grams per square meter of metal deposited in the outermost layer of an aircraft structure subject to a Zone One lightning strike is sufficient to prevent delamination of the aircraft skin. Furthermore, deposition of 100 grams per square meter of metal in an outermost layer of the aircraft subject to a Zone Two lightning strike is sufficient to protect that portion of the aircraft.
In a first prior art method for introducing metal into the fabric of a composite aircraft outermost layer, metal threads are woven into the graphite fabric at regular intervals. While this prior art technique has been proven satisfactory for lightning protection in most cases, it is evident that two different types of fabric having different metal thread counts will be required for zone two and zone 1 areas of an aircraft.
In a second prior art technique for introducing metal into the outermost layer of an aircraft skin, each fiber of the outermost layer is coated with metal prior to being woven into a continuous sheet. This technique is particularly disadvantageous in that the coaxial metal sheath around each fiber has a substantially different modulus of elasticity than the fiber itself. Thus, when the aircraft is subject to bending moments, the metal sheath tends to shear away from the fiber. In addition, unnecessary excess weight is introduced into the fabric weave.
In a third prior art method, described in U.S. Pat. No. 2,042,030, issued to Tainton, one side of a fabric sheet is coated with a relatively thick, metallized layer. This method was not adopted primarily for lightning protection of aircraft but was probably intended to provide a strong, waterproof covering for aircraft in an era when the aircraft fuselage and wings were covered with fabric. In this method, a cathode rotates in an electrolytic solution containing a metal ion to be deposited on one side of the fabric. The rotating cathode is negatively charged and thus attracts a thin coating of metallic ions (such as copper) onto the surface of the cathode. A fabric sheet is then pressed against the rolling cathode, and the metal layer is transferred to the outside of the fabric sheet in a fashion similar to paint being deposited on a wall with a roller. This technique results in the entire side of the fabric being coated with metal, including the interstices between each fiber. A metallized fabric of this type would have a metallized layer which is much thicker than necessary for lightning protection. As previously stated, in a Zone Two strike area, a metal content of approximately 100 grams per square meter is desired. This corresponds to a continuous copper sheet having a thickness of less than 25 .mu.M. The plating technique disclosed in Tainton would not be capable of consistently providing such a thin coating. In addition, interstices in the weave would be filled in, which would disadvantageously interfere with the flex characteristic of the underlying composite material.
Therefore, a need exists for a plating technique which can deposit a very thin layer on one side of a woven fabric sheet and which does not fill in the interstices in the fabric weave.